1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and, more particularly, to an image forming apparatus capable of preventing deterioration in image quality resulting from contamination of a photoconductor due to additive components to a developer, and a method of abrading the photoconductor.
2. Description of Related Art
As electrophotographic developers, heretofore, there are a two-component type developer containing toner and carrier and a single-component type developer containing no carrier. Many image forming apparatuses using such single-component type developer are provided with a friction member for helping frictional electrification of toner with no carrier. However, the charge uniformity of toner is not enough yet and thus an external additive is added to the toner to help to charge.
Recently, the flowability of toner tends to decrease because the toner has a reduced diameter for achieving high quality images in recent years, resulting in an increased specific surface area of the toner. Adding the external additive is also effective in covering the flowability decrease. As the external additive, inorganic compound particles are effective, and desiccated silica is often used. In addition, opposite-charging particles are added in order to enhance movability of toner in an electric field, thereby increasing efficiency of development and transfer. It should be noted that some of the two-component type developers also contain the external additive.
Furthermore, a cleaning member has conventionally been provided for cleaning the surface of a photoconductor. Due to the aforementioned diameter reduction of the toner, the toner is likely to slip or pass through the gap between the cleaning member and the photoconductor. The above external additive also serves as a measure to prevent the toner from slipping or passing through. To be specific, the external additive that leaves from the toner will be accumulated on an edge of the cleaning member, forming an external additive layer. This external additive layer can prevent the passage of the toner.
On the other hand, this may also cause the external additive components to condense and adhere as deposits to the surface of the photoconductor. The external additive components are likely to adhere to the photoconductor particularly under low temperature and low humidity environments. One of mechanisms that generate the above deposits has been known as back-transfer from an intermediate transfer body to the photoconductor. In other words, the external additive transferred onto the intermediate transfer body once will move by one turn of the intermediate transfer body to the transfer point again and transfer back to the photoconductor. Such external additive on the intermediate transfer body has passed or slipped through a secondary transfer point and a cleaning member for the intermediate transfer body and, in many cases, condensed or coagulated on the intermediate transfer body. Thus, the external additive is apt to form deposits.
Even the external additive that has not been transferred forth and back yet, i.e., that remains on the photoconductor, may pass or slip through the cleaning member for the photoconductor and turn along with the photoconductor. Such external additive will fixedly adhere to the photoconductor by electric discharge when it passes through a charging section. Also, charge level of the external additive will change at that time. In the case where the developer containing the opposite-charged particles is used, accordingly, the opposite-charged particles may be attracted by the remaining external additive at the transfer point or the like, resulting in formation of large deposits. This would lead to the occurrence of visible image noises.
JP11(1999)-212293A discloses that an abrasive is added as an external additive in the developer. This abrasive helps to remove the deposits by abrading the surface of the photoconductor while the cleaning member scrapes the toner remaining after transfer. As other techniques, for example, JP9(1997)-244493A discloses that an abrading member is provided in addition to a cleaning member and JP8(1996)-194419A discloses that an abrasive is supplied to a cleaning member.